feat: Settings can re-download the Discord images the None naming broke (3999)

The operator chose a clean re-download over relinking in place. The ~1,600 broken files can't be relinked reliably: their message ids are gone, and their sidecars collided.

Settings → Maintenance → "Repair Discord downloads" previews, then applies:
- Deletes every image whose path is `…/discord/None/<8 digits>_None_…`. Both the folder and the name are required, because that pair is only what the bug produced. It reuses cleanup_service.delete_images for the record and file deletes.
- Sweeps the leftover collided sidecars from those folders and removes the emptied folders.
- Only then clears gallery-dl's archive rows `discord%`, excluding `discordasset_%`. Upstream keys message attachments as `discord{message_id}_{num}`. Since the broken files lost their message ids, per-source forgetting is impossible. Every pre-fix Discord download is broken, and files fetched after the fix still exist on disk, so gallery-dl's `skip` won't re-fetch them.
- Arms a fresh backfill on every Discord source.

The apply defaults to preview at both the route and the task, runs on maintenance_long, and is never on a beat. The card uses the confirm-dialog pattern of AttachmentReclaimCard.

Supporting refactors, with no behaviour change:
- gallery_dl.archive_path() is the single definition of the archive location.
- source_service.arm_backfill() is the mutation start_backfill already did, now shared with the sync repair.

Tests (tests/test_discord_repair.py):
- The archive clear leaves other platforms and Discord assets alone, and counting mutates nothing.
- Case-twin artist folders are both found.
- The folder sweep works.
- An integration run shows only the broken image goes. A correctly named Discord file and a `None` folder under Patreon survive, and only Discord sources are re-armed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

tests/test_discord_repair.py

@@ -0,0 +1,170 @@
+"""The Discord repair (#3999): forget the broken downloads, delete them, backfill again.
+
+The danger in a repair like this is reach. It deletes files and clears
+gallery-dl's memory of what it fetched, so most of these tests pin what it must
+NOT touch: other platforms' archive entries, Discord server assets, images
+outside a `discord/None/` folder, and a correctly named Discord file that
+happens to share a folder name.
+"""
+
+import sqlite3
+
+import pytest
+from sqlalchemy import func, select
+
+from backend.app.models import Artist, ImageRecord, Source
+from backend.app.services import discord_repair
+from backend.app.services.gallery_dl import archive_path
+from backend.app.services.source_service import BACKFILL_MAX_CHUNKS
+
+
+def _archive(tmp_path, entries):
+    path = archive_path(tmp_path)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    conn = sqlite3.connect(path)
+    # The table gallery-dl itself creates (gallery_dl/archive.py).
+    conn.execute("CREATE TABLE archive (entry TEXT PRIMARY KEY) WITHOUT ROWID")
+    conn.executemany("INSERT INTO archive (entry) VALUES (?)", [(e,) for e in entries])
+    conn.commit()
+    conn.close()
+    return path
+
+
+def _entries(path):
+    conn = sqlite3.connect(path)
+    try:
+        return sorted(r[0] for r in conn.execute("SELECT entry FROM archive"))
+    finally:
+        conn.close()
+
+
+# --- the archive ------------------------------------------------------------
+
+
+def test_only_discord_message_entries_are_forgotten(tmp_path):
+    path = _archive(tmp_path, [
+        "discord300000000000000003_1",
+        "discord300000000000000004_2",
+        "discordasset_400000000000000004_9",
+        "hentaifoundry12345",
+    ])
+    assert discord_repair.count_archive_entries(path) == 2
+    assert discord_repair.forget_archive_entries(path) == 2
+    assert _entries(path) == ["discordasset_400000000000000004_9", "hentaifoundry12345"]
+
+
+def test_counting_changes_nothing(tmp_path):
+    path = _archive(tmp_path, ["discord1_1", "discord2_1"])
+    discord_repair.count_archive_entries(path)
+    assert _entries(path) == ["discord1_1", "discord2_1"]
+
+
+def test_a_missing_archive_or_table_is_zero_not_an_error(tmp_path):
+    assert discord_repair.count_archive_entries(tmp_path / "nope.sqlite3") == 0
+    empty = tmp_path / "empty.sqlite3"
+    sqlite3.connect(empty).close()
+    assert discord_repair.forget_archive_entries(empty) == 0
+
+
+# --- the folders ------------------------------------------------------------
+
+
+def test_broken_directories_finds_every_artist_folder_including_case_twins(tmp_path):
+    for artist in ("Conto", "conto", "knuxy"):
+        (tmp_path / artist / "discord" / "None").mkdir(parents=True)
+    (tmp_path / "conto" / "discord" / "general").mkdir(parents=True)
+    (tmp_path / "conto" / "patreon" / "None").mkdir(parents=True)
+    found = [
+        p.relative_to(tmp_path).as_posix()
+        for p in discord_repair.broken_directories(tmp_path)
+    ]
+    assert found == ["Conto/discord/None", "conto/discord/None", "knuxy/discord/None"]
+
+
+def test_sweep_removes_leftover_sidecars_and_the_empty_folder(tmp_path):
+    d = tmp_path / "conto" / "discord" / "None"
+    d.mkdir(parents=True)
+    (d / "image.json").write_text("{}")
+    (d / "20240716_None_rejected.png").write_bytes(b"x")
+    removed, gone = discord_repair._sweep_directory(d)
+    assert (removed, gone) == (2, True)
+    assert not d.exists()
+
+
+# --- the whole repair, against the database -----------------------------------
+
+
+def _image(db_sync, artist, path, sha):
+    img = ImageRecord(
+        artist_id=artist.id, path=str(path), sha256=sha * 64, size_bytes=100,
+        mime="image/png", origin="downloaded",
+    )
+    db_sync.add(img)
+    return img
+
+
+@pytest.mark.integration
+def test_repair_deletes_only_broken_discord_images_and_rearms_discord_backfills(
+    db_sync, tmp_path,
+):
+    artist = Artist(name="Conto", slug="conto")
+    db_sync.add(artist)
+    db_sync.flush()
+    discord = Source(
+        artist_id=artist.id, platform="discord", url="https://discord.com/channels/1/2",
+        enabled=True, config_overrides={"_backfill_state": "complete", "_backfill_cursor": "x"},
+    )
+    patreon = Source(
+        artist_id=artist.id, platform="patreon", url="https://www.patreon.com/conto",
+        enabled=True, config_overrides={},
+    )
+    db_sync.add_all([discord, patreon])
+
+    broken_dir = tmp_path / "conto" / "discord" / "None"
+    good_dir = tmp_path / "conto" / "discord" / "general"
+    patreon_dir = tmp_path / "conto" / "patreon" / "None"
+    for d in (broken_dir, good_dir, patreon_dir):
+        d.mkdir(parents=True)
+    broken = broken_dir / "20240716_None_image.png"
+    good = good_dir / "20240716_300000000000000003_01_image.png"
+    # A `None` folder under another platform, and a `_None_` name outside one.
+    # Neither is the bug's pair, so neither may be touched.
+    elsewhere = patreon_dir / "20240716_None_image.png"
+    for f in (broken, good, elsewhere):
+        f.write_bytes(b"x")
+    (broken_dir / "image.json").write_text("{}")
+    _image(db_sync, artist, broken, "a")
+    _image(db_sync, artist, good, "b")
+    _image(db_sync, artist, elsewhere, "c")
+    db_sync.commit()
+    _archive(tmp_path, ["discord300000000000000003_1", "hentaifoundry1"])
+
+    preview = discord_repair.repair_discord_downloads(db_sync, images_root=tmp_path, dry_run=True)
+    assert preview["images"] == 1
+    assert preview["archive_entries"] == 1
+    assert broken.exists()
+
+    result = discord_repair.repair_discord_downloads(db_sync, images_root=tmp_path, dry_run=False)
+    assert result["images_deleted"] == 1
+    assert result["remaining"] == 0
+    assert result["backfills_started"] == 1
+
+    assert not broken.exists() and not broken_dir.exists()
+    assert good.exists() and elsewhere.exists()
+    assert _entries(archive_path(tmp_path)) == ["hentaifoundry1"]
+
+    kept = db_sync.execute(select(func.count(ImageRecord.id))).scalar_one()
+    assert kept == 2
+
+    rows = {
+        platform: (runs, overrides)
+        for platform, runs, overrides in db_sync.execute(
+            select(Source.platform, Source.backfill_runs_remaining, Source.config_overrides)
+        ).all()
+    }
+    runs, overrides = rows["discord"]
+    assert runs == BACKFILL_MAX_CHUNKS
+    assert overrides["_backfill_state"] == "running"
+    assert "_backfill_cursor" not in overrides
+    # Only Discord sources are re-armed.
+    assert rows["patreon"][1] == {}